docs: add flow user guide

docs/nightly/en/summary.yml

@@ -52,6 +52,12 @@
  - sql
  - promql
  - query-external-data
+ - Continuous-Aggregation:
+ - overview
+ - manage-flow
+ - define-time-window
+ - query
+ - expression
  - Client-Libraries:
  - overview
  - go

docs/nightly/en/user-guide/continuous-aggregation/define-time-window.md

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+# Define Time Window
+
+Time window is an important attribute of your continuous aggregation query. It defines how the data is aggregated in the flow. GreptimeDB provides two types of time windows: `hop` and `tumble`, or "sliding window" and "fixed window" in other words. You can specify the time window in the `GROUP BY` clause using `hop()` function or `tumble()` function respectively. These two functions are only supported in continuous aggregate queries's `GROUP BY` position.
+
+# Tumble
+
+`tumble()` defines fixed windows that do not overlap. It signature is like the following:
+
+```
+tumble(col, interval, <start_time>)
+```
+
+Where `col` specifies use which column to compute the time window. The provided column must have a timestamp type. `interval` specifies the size of the window. The `tumble` function divides the time column into fixed-size windows and aggregates the data in each window.
+
+`start_time` is an optional parameter to specify the start time of the first window. If not provided, the start time will be aligned to calender.
+
+# Hop (not supported yet)
+
+`hop` defines sliding window that moves forward by a fixed interval. It signature is like the following:
+
+```
+hop(col, size_interval, hop_interval, <start_time>)
+```
+
+Where `col` specifies use which column to compute the time window. The provided column must have a timestamp type.
+
+`size_interval` specifies the size of each window, while `hop_interval` specifies the delta between two windows' start timestamp. You can think the `tumble()` function as a special case of `hop()` function where the `size_interval` and `hop_interval` are the same.
+
+`start_time` is an optional parameter to specify the start time of the first window. If not provided, the start time will be aligned to calender.

docs/nightly/en/user-guide/continuous-aggregation/expression.md

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+# Expression
+
+This part list all supported aggregate functions.
+
+- `count(column)`: count the number of rows.
+- `sum(column)`: sum the values of the column.
+- `avg(column)`: calculate the average value of the column.
+- `min(column)`: find the minimum value of the column.
+- `max(column)`: find the maximum value of the column.

docs/nightly/en/user-guide/continuous-aggregation/manage-flow.md

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+# Manage Flows
+
+Each `flow` is a continuous aggregation query in GreptimeDB. It is a query that continuously updates the aggregated data based on the incoming data and materializes the result. This document describes how to create, update, and delete a flow.
+
+A `flow` have those attributes:
+- `name`: the name of the flow. It's an unique identifier in the catalog level.
+- `source tables`: tables provide data for the flow. Each flow can have multiple source tables.
+- `sink table`: the table to store the materialized aggregated data.
+- `expire after`: the interval to expire the data from the source table. Data after the expiration time will not be used in the flow.
+- `comment`: the description of the flow.
+- `SQL`: the continuous aggregation query to define the flow. Refer to [Expression](./expression.md) for the available expressions.
+
+# Create or update a flow
+
+The grammar to create a flow is:
+
+```sql
+CREATE [ OR REPLACE ] FLOW [ IF NOT EXISTS ] <name>
+OUTPUT TO <sink-table-name>
+[ EXPIRE AFTER <expr> ]
+[ COMMENT = "<string>" ]
+AS 
+<SQL>;
+```
+
+When `OR REPLACE` is specified, if a flow with the same name already exists, it will be updated to the new one.
+
+`sink-table-name` is the table name to store the materialized aggregated data. It can be an existing table or a new table, `flow` will create the sink table if it doesn't exist. But if the table already exists, the schema of the table must match the schema of the query result.
+
+`SQL` part defines the continuous aggregation query. Refer to [Query](./query.md) for the details. Generally speaking, the `SQL` part is just like a normal `SELECT` clause with a few difference.
+
+A simple example to create a flow:
+
+```sql
+CREATE FLOW IF NOT EXISTS my_flow
+OUTPUT TO my_sink_table
+EXPIRE AFTER INTERVAL '1 hour'
+COMMENT = "My first flow in GreptimeDB"
+AS
+SELECT count(item) from my_source_table GROUP BY tumble(time_index, INTERVAL '5 minutes');
+```
+
+The created flow will compute `count(item)` for every 5 minutes and store the result in `my_sink_table`. All data comes within 1 hour will be used in the flow. For the `tumble()` function, refer to [define time window](./define-time-window.md) part.
+
+# Delete a flow
+
+To delete a flow, use the following `DROP FLOW` clause:
+
+```sql
+DROP FLOW [IF EXISTS] <name>
+```
+
+For example:
+
+```sql
+DROP FLOW IF EXISTS my_flow;
+```

docs/nightly/en/user-guide/continuous-aggregation/overview.md

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+# Overview
+
+GreptimeDB provides a continuous aggregation feature that allows you to aggregate data in real-time. This feature is useful when you need to calculate and query the sum, average, or other aggregations on the fly. The continuous aggregation feature is provided by our `Flow` engine. It continuously updates the aggregated data based on the incoming data and materialize it.
+
+- [Manage Flow](./manage-flow.md) describes how to create, update, and delete a flow. Each of your continuous aggregation query is a flow.
+- [Define Time Window](./define-time-window.md) describes how to define the time window for the continuous aggregation. Time window is an important attribute of your continuous aggregation query. It defines the time interval for the aggregation.
+- [Query](./query.md) describes how to write a continuous aggregation query.
+- [Expression](./expression.md) is a reference of available expressions in the continuous aggregation query.
+
+![Continuous Aggregation](/flow-ani.svg)

docs/nightly/en/user-guide/continuous-aggregation/query.md

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+# Write a Query
+
+This chapter describes how to write a continuous aggregation query in GreptimeDB. Query here should be a `SELECT` statement with either aggregating functions or non-aggregating functions (i.e., scalar function).
+
+Only two kinds of expression are allowed after `SELECT` keyword:
+- Aggregate functions: see the reference in [Expression](./expression.md) for detail.
+- Scalar functions: like `col`, `to_lowercase(col)`, `col + 1`, etc. This part is the same as the normal `SELECT` clause in GreptimeDB.
+
+Then each query should have a `FROM` clause to specify the source table. The referenced source table should be one in the flow's source tables in `CREATE FLOW` clause. Join is currently not supported so each query can reference only one table.
+
+`GROUP BY` clause works as in a normal query. It groups the data by the specified columns. One special thing is the time window functions `hop()` and `tumble()` described in [Define Time Window](./define-time-window.md) part. They are used in the `GROUP BY` clause to define the time window for the aggregation. Other expressions in `GROUP BY` can be either literal, column or scalar expressions.
+
+Notice the two time window functions will add several columns to the output schema. The columns are:
+- `window_start`: the start time of the window.
+- `window_end`: the end time of the window.
+- `updated_at`: the time when the window is updated.
+
+Others things like `ORDER BY`, `LIMIT`, `OFFSET` are not supported in the continuous aggregation query.